The primary structure of human tissue amyloid P component from a patient with primary idiopathic amyloidosis.

The amino acid sequence of human tissue amyloid P component (AP) extracted by a modified method from the spleen of a patient with primary idiopathic amyloidosis was determined. AP is a glycoprotein composed of a pair of noncovalently bound pentameric discs with a subunit size of 23-25 kDa. Each subunit consists of 204 residues, a single disulfide bridge linking Cys 36 to Cys 95, and a carbohydrate moiety attached to Asn 32. The precursor of AP is the serum amyloid protein (SAP). The primary structure of AP presented here differs from the amino acid sequence of SAP previously reported, but is identical to the amino acid sequence of mature SAP deduced from the nucleotide sequence of complementary DNA clones. It shares 52% homology with the amended sequence of human C-reactive protein, an acute phase protein, and 68% homology with the Syrian hamster "female protein," another acute phase protein whose response is modulated by sex steroids. AP/SAP, C-reactive protein, and female protein belong to a family of plasma proteins called pentraxins and their considerable sequence homology is probably the result of gene duplication. Neither the physiological function of AP nor its possible pathological role in amyloidosis are yet known.